Heading and Course over the Ground (COG) vectors are two of the most powerful tools offered by electronic navigation.
By combining the computing power of a chart plotter with data from satellites and onboard sensors, they provide navigators with much of the basic information for safe passage planning.
A vector is something that has a numerical value and direction.
Heading and COG vectors use speed to calculate distance and course to project a future position. Whilst vector length is usually defined by time, some systems use fixed lengths. Figure 1 shows two vectors: a red heading vector, and a green COG vector.
Figure 1
The heading vector shows the bearing of the vessel’s head, or bow, relative to North. It’s usually displayed in degrees from 000° (North) to 359° and the input for this comes from a heading sensor fitted to the vessel.
The length of a heading vector is derived from the vessel’s speed through the water using input from a water speed log. In figure 1, the time set for the vector is 12 minutes (bottom-left of the screen), meaning the end of the vector is where the vessel would be in 12 minutes’ time if nothing else was affecting the vessel’s course, such as wind. The vector moves with the vessel and only alters length or direction in line with changes in heading and/or speed.
It’s vital that the navigator understands that the vessel will almost certainly not travel in the projected direction, as it’s only taking the heading and speed through the water into account, and not allowing for any impact of water currents or leeway. Heading vectors are considered to be water- stabilised as their inputs are taken from speed and direction through the water.
“Most electronic chart systems aren’t able to make a judgement – that’s for the humans using the equipment”
COG vectors use COG and Speed Over Ground (SOG) to project the motion of the vessel over the Earth’s surface. This means that COG vectors are considered as ground- stabilised, as they use speed and direction over the ground. Both SOG and COG are derived from a Global Navigation Satellite System (GNSS) receiver and show the vessel’s movement after any effect of wind and/or current. In figure 1, the vector is set for 12 minutes, suggesting the vessel will have moved to where the green arrow is pointing. COG vectors change length and direction when heading, speed, tidal streams, or wind changes.
Although there are self-driving cars and ships, most electronic chart systems (ECS) can only present the data they access and don’t have the ability to make a judgement. The human using the equipment still needs to be in charge and make the decisions.
The example screen shows a black dotted line denoting the Bearing to Waypoint (BTW), which is 106°T. The navigator must match the COG vector with the dotted line or, to put it another way, point the COG vector at the waypoint. This would require an alteration to port. It can be estimated as 10° based on the difference between the COG value at the top of the screen and the BTW value at the bottom of the screen. The course to steer can be fine tuned from the first estimate.
Not all ECSs will be installed with a heading sensor and water speed input to enable a heading vector. The good news is that the COG vector will work for monitoring progress with just a GNSS input. Without a heading sensor, the ECS will not be able to stabilise a radar display to an accurate North or course up, although it would get an approximate North by using the COG as an approximate heading, but only when the vessel is moving. The error would be greater in slow-moving vessels in strong tidal streams and strong winds.
The RYA is working closely with the UKHO and Maritime and Coastguard Agency (MCA) to represent the views of our members on the production of paper charts. Find out more about the latest updates and keep an eye out for further information.
For further reading, the new edition of RYA Day Skipper Shorebased Notes is available to pre-order and covers the balance of paper-based and digital techniques for navigation.
Article first shared to members in the autumn 2024 edition of RYA magazine.